Previously the test knew how to find an OpenLDAP installation at the
paths used by Homebrew.  Add the MacPorts paths too.

Author: Thomas Munro
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/CA%2BhUKGKrjGS7sO4jc53gp3qipCtEvThtdP_%3DzoixgX5ZBq4Nbw%40mail.gmail.com